Separate Markdown Style Guidelines from General Guidelines

[mcdonnnj]

💡 Summary

Break out all of the Markdown specific guidelines in style-guide.md into an appropriate languages/markdown section.

Motivation and context

Currently the information in style-guide.md contains plain language guidelines intermixed with Markdown specific guidelines. I believe it would be better served to break out any Markdown specific guidelines into it's own language section. This will make it more obvious where to find the information as well as improving maintainability.

Implementation notes

The PR should be limited to creating a new languages/markdown section and only moving existing Markdown content in style-guide.md to this section. Any additions/modifications to the Markdown guidelines should be kept to their own PRs after this separation.

Acceptance criteria

• [ ] Create the PR
• [ ] Everyone approves the PR